Abstract
A secure taper connection in shoulder arthroplasty is mandatory to avoid loosening and fretting. This study's objective was to determine the amount of in situ force used by surgeons to seat a humeral head and to determine the disengagement force of the taper connection. The influence of 1) material type, 2) head size, and 3) surgeon on the impaction force and the fixation (pulloff force) of the sample was examined.
Methods
Impaction data was collected from experienced shoulder surgeons (n=5) during a cadaver lab. Testing groups (n=5 each) were: 1) small ceramic, 2) big ceramic, 3)small metal and 4) large metal. Twenty centric, anatomic humeral heads (DJO surgical, Vista, CA, TURONTM, material: CoCrMo or BIOLOX®delta, size: 38×14mm or 54×22mm) were paired with a corresponding humeral neck (TURON™, DJO surgical, type: neutral modular, material: CoCrMo). Each taper was always used with the same humeral head throughout testing.
The impaction force sequence was recorded using an instrumented impactor (Piezo sensor, model 208 C05, PCB PIEZOTRONICSINC, Depew, NY, ±1%). The surgeons impacted all samples into the cadaver using their typical pattern of hammer strikes (Figure 1). The engaged humeral head and taper were removed by hand and then disengaged using an instrumented (U93, HBM, Darmstadt, Germany, load limit: 5kN) hand-held pulloff-device.
Statistics and data analysis were performed in MATLAB (2014b, Mathworks, Natick, MA, α=0.05). Two-tailed, pearson's linear correlation coefficients are reported. Group differences were determined using Kruskal Wallis test. Pair-wise comparisons were performed using a Tukey correction.
Results
Extremely high and variable impaction forces were measured (Table 1, Figure 2). The maximum force was nearly 27 kN; however, that value reduced to ∼18kN when the data from an outlier surgeon was removed. Maximum impaction forces were 12.45±4.36 kN, and the average was 10.47±3.63 kN. The pulloff force ranged from 0.94 kN to 5.54 kN with an average of 2.76±1.19 kN. Higher impaction forces required higher pulloff forces to disengage the taper connection (p<0.001, R>−0.608).
Ceramic humeral heads showed a 24% higher fixation strength (p=0.004) under similar engagement conditions (p=0.18) in comparison to metal components.
Head size does not appear to influence either the magnitude of the impaction force surgeons use (p>0.20) nor the force needed to disengage the taper (p=0.25).
The surgeon performing the insertion had a significant influence on the impaction strike timing (p<0.001), number of strikes (p<0.001), and the impaction forces (p<0.03) and the pulloff force (p<0.001).
Conclusions
Impaction forces were markedly larger than those recorded for taper engagement in hip arthroplasty. The ceramic humeral component showed greater fixation strength in comparison to the metal for similar impaction forces. Pulloff forces were approximately 25% of the impaction force. Potentially, this low taper efficiency resulted from the cadaver absorbing much of the energy rather than the taper connection. The influence of the patient and the clinical situation on the taper efficiency is unknown. Variations between surgeons greatly influenced the impaction and the fixation force. Therefore, individual surgeon practices may substantially influence clinical fixation strength of tapered shoulder implants.
